The present invention relates to a water pump bearing.
A conventional water pump bearing unit as shown in FIG. 4 (Utility Model Application (OPI) No. 172116/83) is equipped with an impeller on one end of a water pump shaft 1 and a pulley on the other. An outer ring 2 is arranged around the outer periphery of the shaft 1 between the impeller and the pulley. A plurality of balls 3, are located in the annular space between the shaft 1 and the outer ring 2 on the impeller side and a plurality of rollers 5, are located on the pulley side. Moreover, the outer peripheries 7a of seal members 7 are fitted to annular stepped portions 6 provided on the inner peripheral surfaces 2a of both ends of the outer ring 2, respectively. Further, bidirectional seal lips 7b-1, 7b-2 are formed on the inner peripheries 7b of the seal members 7 extend back and forth in the axial direction so as to rub the outer peripheral surface 1a of the shaft 1. The seal members 7 are used as bidirectional seals on both sides of the outer ring 2 in the axial direction.
The difficulty with the aforesaid conventional water pump bearing unit is if the pressure in the annular space changes such that the temperature drops while the water pump stops operating, both the seal members 7 change negatively because of volumetric change of a grease or air in the space thus the negative pressure state is maintained because toth the seal members 7 have bidirectional sealing properties. As a consequence, water may enter the water pump bearing unit through the contact between the seal member 7 on the impeller side and the shaft 1. The foreign substance causes grease to deteriorate and permits not only seizing but also rust generation therein.